Atomic clocks use the frequency of the electronic transition of an alkali metal vapor as a frequency reference. Alkali metal gasses, such as Cesium, Rubidium or other atom with a single electron in the outer shell, undergo optical transitions at very high discrete frequencies in the hundreds of GHz (optical wavelengths of around 800-900 nm). Atomic clocks determine the frequency of the electronic transition of a vaporized alkali atom by optically interrogating the gas over a bandwidth including the transition frequency, with the absorption detected at the transition frequency identifying the absolute frequency reference for the clock. Chip-scale Alkali vapor atomic clocks typically use an optical transparency peak (e.g., coherent population trapping) verses an absorption null (incoherent microwave pumping) to lock a reference frequency. Such electronic transition clocks, however, require thermal stability of the laser optical source and the electronic transition vapor cell itself requires stable gas temperature, and heating circuitry is therefore often needed. Electronic transition clocks include a modulator to modulate the laser signal, and multiple complex electronic control loops are required for operation. Also, electronic transition atomic clocks typically require a coil around the cell or other magnetic shielding to shield external magnetic fields to provide a constant magnetic field at the physics cell inside the shield in order to break degeneracy of ground-state Zeeman levels. Accordingly, electronic transition clocks suffer from relatively high power consumption, as well as additional cost and space for the necessary circuitry including the laser, modulator, photo detector and other optical components such as collimators, isolators, polarizers, lenses, etc.